1. Field of the Invention
This invention relates to a process wherein the residue, which is obtained from the distillation of the product of phosgenating toluene diamine to toluene diisocyanate, is used to produce dihydroxytoluene.
Dihydroxytoluene is a known chemical which is used in certain medicinal applications and as a reagent for carbohydrates. It is also of potential utility in the production of plastic resins.
Toluene diisocyanate is produced on a large commercial scale by a process which comprises reacting toluene diamine with excess phosgene usually in the presence of an organic solvent medium. An illustrative process is disclosed in U.S. Pat. No. 3,287,387. Along with toluene diisocyanate, the phosgenation product mixture usually comprises unreacted phosgene, solvent, hydrogen chloride by-product, and a relatively substantial proportion of side reaction products in the form of residual and high-boiling materials.
Recovery of a substantial or major proportion of toluene diisocyanate from this mixture is achieved by distillation which is usually effected in two or more steps to enable removal of the low-boiling components, e.g., the phosgene and solvent first before recovering the toluene diisocyanate. The remaining residue from distillation is a solid or viscous mixture which is often discarded.
2. Description of the Prior Art
In order to recover toluene diamine values from the residue of the distillation of the product of phosgenating toluene diamine, U.S. Pat. Nos. 3,128,310 and 3,331,876 disclose the concept of reacting the residue with water in the presence of alkali.
U.S. Pat. No. 3,462,497 discloses the use of phosphoric acid in the hydrolysis of phenylene diamine to resorcinol; and U.S. Pat. No. 3,933,925 discloses the hydrolysis of toluene diamine to methyl resorcinol in the presence of ammonium bisulfate.
Further in the prior art, U.S. Pat. No. 3,799,963 discloses a process for reducing the acidity and hydrolyzable chloride content of toluene diisocyanate by heating it in the presence of formic acid, and U.S. Pat. No. 3,350,438 discloses the preparation of biuret polyisocyanates by the reaction of organic polyisocyanates with formic acid.